New Greenhouse build with thermal mass storage; air-to-water heat exchange system

Not sure whether I am understanding correctly.  My own 16 X 12 ft greenhouse has above ground water tank storage against the (insulated) back wall, 16" deep X 28" high X 12 ft. , with plywood covers which we use for seed starting in spring. This does not store enough BTUs to maintain the air temp at anything better than 5-10 degrees C at night, with occasional dips to -2 degrees C.  (But the soil temp never fell lower than +8 C all winter.  The water circulates up to car radiators in the peak when the air temp exceeds the water temp, (during the day), and drains down when the pumps turn off with falling air temp.  (So no concerns about freezing in radiators or supply lines). In addition, the water circulates through the grow beds continuously, (separate set of pumps, running 24/7).  The temperature of the water returning from the radiators runs, on average, ~ 2 degrees C higher that the water going up. The temperature of the soil in the grow beds may run up to 2 degrees C lower than the water temperature after prolonged periods of dull weather, but did not fall below 8 degrees at any point during this past winter.  (FWIW, during the coldest period, as long as we got a day of sun, it rose to ~ 12-13 degrees), slowly falling back over the succeeding several dull days to a nadir of 8 degrees.)  The whole affair is passive solar - no supplementary heat source, (but does use 12-V power for the pumps, fans, and controllers.)

In practice, the limiting factor for growing lettuce through the winter has not been the temperatures, but the lack of light.  (Lettuce needs a minimum of 10 hrs. of light daily, the so-called Persephone Point.)  Swiss chard grew right through the winter.  (And we are not into arugola or other cold-tolerant greens.)

David Maxwell, Zone 5b, Nova Scotia, Canada

do you think you could capture and store more heat if you had more water?  The dimensions you listed for your water tank equals 280 gallons.  I'm proposing about 2x that, 500 gallons or more

based on a passive solar greenhouse book from the 70s I was reading, recommending a rule-of-thumb for thermal mass: 2-3 gallons of water per square foot of greenhouse.
So 12'x16' x 3 gallons = 576 gallons of water

Absolutely!  I do not have nearly enough water.  And if I were doing it over again, I would somehow wangle a lot more water storage, (but I still want some space to grow plants...), OR ,(and I think this might have been a better choice in the first -place), a GAHT system, which is both more efficient at capturing heat rapidly, and stores 100 times more energy per cu. ft. of storage, (because of the way it works, depending on the heat of evaporation rather than simple thermal storage).

yes the "wasted space" issue is a hurdle.  The "wall of black oil drums" always bothered me; both ugly and taking up otherwise productive growing space

there is a company selling geodesic dome greenhouses with above-ground ponds: https://growingspaces.com/geodesic-dome-greenhouse/thermal-mass/
(don't look at the price tag or you'll have a heart attack!)
They're touting the pond as a place to grow water plants, or koi fish, etc.  
You could alternatively use it as a large wicking bed, or floating raft hydroponic or aquaponic system

My point being that the water can be both thermal mass and productive growing area, if designed such from the beginning